In the rich and complex visual environment that surrounds us, visual stimuli compete for attention in a limited capacity perceptual system (Broadbent, 1958; Duncan, 1980; Treisman, 1969). In this competition, the winners reach perceptual awareness and the losers are disregarded and fail to reach awareness (Ward, Goodrich & Driver, 1994; Mattingley, Davis & Driver, 1997). Theories of visual attention can be guided and informed by the study of brain damaged patients who show specific impairments in attending to visual stimuli, in particular visual extinction, commonly following right hemisphere damage and resulting in an inability to perceive a contralesional stimulus when it appears with a simultaneous ipsilesional item, but no such impairment when it appears alone. The studies reported in this thesis created an extinction-like pattern of errors in healthy volunteers using a bottom-up (stimulus- driven) paradigm when a simple task of detection was employed. When a more demanding task of stimulus identification employed, both in bottom-up and top-down (cueing) paradigms, a rarely previously described pattern of anti-extinction was observed, in which perception of a weaker item was facilitated (rather than impaired) by a simultaneous ‘stronger’ item in the display. Extinction and anti-extinction were then explored in brain damaged patients. A novel ‘attentional waiting’ hypothesis was discussed, which proposes that extinction and anti-extinction may be part of the same attentional mechanism where the latter manifestation may be observed in larger proportion of patients showing extinction if duration of stimuli is increased.